Field of the Invention
The present invention relates to an image forming apparatus, an image forming system, a method of calibrating an image detection unit, and a post-processing apparatus for detecting an image formed on a transfer medium.
Description of the Related Art
In an image forming apparatus such as a copying machine, a fax machine, a printer, and a multifunction printer, an image is formed on a transfer paper by an image forming unit based on image data. Such an image forming process is performed according to an image forming condition (position, color, density and the like for the image forming) based on the image data. As the image forming apparatus is operated, the image forming condition can change temporarily or gradually, so quality is maintained through various kinds of an automatic adjustment function and the like.
For example, a patch image or the like is formed on a transfer paper at intervals of predetermined time of the operation of the image forming apparatus or predetermined number of printed sheets. The patch image is read by a line sensor or the like, and then the state of the image and the image forming condition are compared to determine if the image is formed properly. If the image is not properly formed according to the image forming condition and there is misalignment, calibration or the like is made on the image forming condition to maintain the quality of the image.
For example, in Japanese Patent Laid-Open No. 2007-30340, an image is read by a sensor apparatus including a colorimeter or a densitometer, and then density of a toner image formed by an image forming unit is adjusted according to the read result. Further, in Japanese Patent Laid-Open No. 2007-30340, if there is a stain on a part of the patch, a patch image is formed and read for an area excluding the area with the stain so as to be able to adjust output density.
For reading of an image, a line sensor using a CCD or a CIS as an image capturing element is widely used. The line sensor is suitable for high-speed reading and can read a fixed transfer paper at conveying speed of a fixing part, and then inspection can be made for color reproducibility, print positioning and the like. However, the line sensor is not suitable for color reproducibility, and therefore it is preferable that the line sensor is subject to periodic calibration using a spectral colorimeter for which colorimetric accuracy is ensured.
However, when calibration of the line sensor is performed with a spectral colorimeter in a configuration where the spectral colorimeter and the line sensor are provided on the same conveying path, a patch on the same chart needs be read. The spectral colorimeter can only measure a color as an average value in a spot area of a patch on the chart, and cannot determine if the patch is properly printed. The problem is that the spectral colorimeter cannot accurately measure colors of a patch when there is uneven density, a stain, a scratch or the like on the chart and therefore cannot properly perform calibration of the line sensor.
The present invention is made in view of the background above. One object of the present invention is to read an image and checks whether there is uneven density, a stain, a scratch and the like in order to determine print quality by a first image detection unit, and read by a second image detection unit the image determined as its printing quality being OK in order to calibrate the first image detection unit.